Oxytocin, which is nicknamed by some the "cuddle hormone" or the "love hormone" is actually more properly identified as a neuropeptide because it acts not just within your body but also within your brain. Oxytocin has long been known to play a key role within social bonding and attachment.

Clear evidence of this first emerged from experiments with a monogamous breed of prairie voles: Oxytocin, when dripped into one animal's brain in the presence of the opposite sex, creates a long-lasting preference to remain together, cuddled up side-by-side, behavior taken as evidence that oxytocin sparked the formation of a powerful social bond between them. In humans, oxytocin surges during sexual intercourse for both men and women, and, for women, during childbirth and lactation, pivotal interpersonal moments that stand to forge new social bonds or cement existing ones. The natural blasts of oxytocin during such moments are so large and powerful that for many years they all but blinded scientists to the more subtle ebb and flow of oxytocin during more typical day-to-day activities, like playing with your kids, getting to know your new neighbor, or striking a deal with a new business partner. Technical obstacles also needed to be cleared.

Decades after oxytocin's role in monogamous prairie voles had been amply charted, scientists studying human biochemistry still struggled to find ways to reliably and noninvasively measure and manipulate oxytocin during natural behavior. Scientific understanding of oxytocin's role in your everyday social life could not advance without more practical research tools at hand.

Dramatic new evidence on oxytocin's power to shape your social life first surfaced in Europe, where laws permitted the use of a synthetic form of oxytocin, available as a nasal spray, for investigational purposes. Among the first of these studies was one in which 128 men from Zurich played the so-called "trust game" with real monetary outcomes on the line. At random, these men were assigned to either the role of "investor" or the role of "trustee," and each was given an equivalent pot of starting funds. Investors made the first move in the game. They could give some, all, or none of their allocated funds to the trustee. During the transfer of funds, the experimenter tripled their investment while letting the trustee know how much the investor had originally transferred. Trustees made the next move. They could give some, all, or none of their new allotment of funds (the investors' tripled investment plus their own original allocation) back to investors. The structure of the game puts investors, but not trustees, at risk. If an investor chose to entrust the other guy with their investment, he risked receiving nothing in return if the trustee chose to selfishly keep the entire monetary gain to himself. But if the trustee was fair, they could each double their
money.

Prior to playing this trust game, using a double-blind research design, 15 participants received either oxytocin or an inert placebo by nasal spray. The effect of this single intranasal blast of oxytocin on the outcome of the trust game was dramatic: The number of investors who trusted their entire allotment to their trustee more than doubled.

Interestingly, the simple act of sharing an important secret from your life with someone you just met increases your naturally circulating levels of oxytocin, which in turn raises your confidence that you can trust that person to guard your privacy. Thankfully, we also know that oxytocin does not induce trust indiscriminately, making people gullible
and therefore open to exploitation. The effects of oxytocin on trust turn out to be quite sensitive to interpersonal cues, like those subtle signs that tip you off that another may be the gambling type, or irresponsible in other ways.

Researchers have since moved on to examine the effects of oxytocin on people's sensitivities to the subtle social cues that signal whether or not trust is warranted. From this work, I can tell you that under the influence of oxytocin, you attend more to people's eyes and become specifically more attuned to their smiles, especially subtle ones. Perhaps because of the closer attention you pay to peoples' smiles and eyes, you become a better judge of their feelings, and view people on the whole as more attractive and trustworthy. You also become particularly sensitized to environmental cues linked to positive social connections — for instance, to words like "love" and "kissing."

More generally, oxytocin is has been cast as a lead character within the mammalian "calm-and-connect" response, a distinct cascade of brain and body responses best contrasted to the far more familiar "fight-or-flight" response. Let's face it, meeting new people can be a little scary at times. Think back to what it was like for you on your first day at a new school or in a new job. You're suddenly thrown in with people you'd never heard of before. Even if a new person seems friendly, it's hard to know his true motives. Oxytocin appears both to calm fears that might steer you away from interacting with strangers and also to sharpen your skills for connection.

As I've mentioned, though, oxytocin is far from blind. It indeed heightens your attunement to cues that signal whether others are sincere or not. Through eye contact and close attention to all manner of smiles — and the embodied simulations such visual intake triggers — your gut instincts about who to trust and who not to trust become more reliable. Rather than avoid all new people out of fear and suspicion, oxytocin helps you pick up on cues that signal another person's good will, and guides you to approach them with your own. Because all people need social connections, not just to reproduce, but also to survive and thrive in this world, oxytocin has been dubbed "the great facilitator of life." It too can jump the gap between people such that someone else's oxytocin flow can trigger your own. A biochemical synchrony can then emerge that supports mutual engagement, care and responsiveness.

It turns out that positive behavioral synchrony — the degree to which an infant and a parent, through eye contact and affectionate touch, laugh, smile, and coo together — goes hand-in-hand with oxytocin synchrony. Researchers have measured oxytocin levels in the saliva of dads, moms, and infants both before and after a videotaped, face-to-face parent-infant interaction. For infant-parent pairs who show mutual positive engagement, oxytocin levels also come into sync. Without such engagement, however, no oxytocin synchrony emerges. Positivity resonance, then, can be viewed as the doorway through which the exquisitely attuned biochemical tendencies of one generation influence those of the next generation to form lasting, often lifelong bonds. Knowing, too, that oxytocin can ebb and flow in unison among non-kin — even among brand-new acquaintances just then learning to trust one another — micro-moments of love, of positivity resonance, can also be viewed as the doorways through which caring and compassionate communities are forged. Love, we know, builds lasting resources. Oxytocin, studies show, swings the hammer.

This article has been adapted by arrangement with Hudson Street Press, a member of Penguin Group (USA) Inc., from LOVE 2.0by Barbara Fredrickson M.D. Copyright 2013 by Barbara Fredrickson Ph.D.